In recent years, there has been an upsurge of interest in so-called renewable energy resources, particularly carbohydrates. Carbohydrates can be converted to liquid organic fuels in a number of different ways. For example, simple sugars have been fermented to produce ethyl alcohol since ancient times. If liquid organic fuels obtained from carbohydrates are to be economically competitive with other energy sources, however, it is likely that starting materials for the fermentation or other microbiologically or enzymatically catalyzed syntheses will have to be selected from more complex carbohydrates or carbohydrate-containing materials, particularly materials of a cellulosic nature.
Enzymes and microorganisms capable of breaking down cellulose and hemicelluloses into mono- and oligosaccharides are known. In some instances, the same microorganism culture which breaks down the cellulose will ferment the simple sugar intermediates, thereby providing an essentially one-step conversion of cellulose to liquid organic solvents or fuels. (In actuality, of course, enzymatically or microbiologically catalyzed hydrolysis of cellulose and fermentation of the resulting simple sugars to alcohols, ketones, and the like is an extremely complex series of reactions involving many intermediates, but some of these intermediates or theoretically postulated intermediates, e.g. glyceraldehyde have only a short-term existence and do not show up in the ultimately obtained fermentation products.)
The scientific literature dealing with the utilization of cellulosic materials by microorganisms is very large. According to one study, cellulose-decomposing bacteria can be divided into three groups: (a) aerobic, (b) anaerobic, and (c) thermophilic. Very few aerobic bacteria have been used successfully for the purpose of deriving fuels or solvents from a cellulosic raw material. According to Porter, Bacterial Chemistry and Physiology, Wiley and Sons, Inc., New York, N.Y., 1946, page 822, "Aerobic bacteria and fungi usually bring about complete destruction of cellulose, without leaving much in the form of intermediate products; hence they have little to offer for the production of industrially valuable products." A disadvantage with thermophilic bacteria is that they tend to be inefficient or even ineffective at normal ambient or modestly elevated temperatures. In short, the use of thermophilic bacteria for decomposition of cellulose actually involves a significant energy input beyond the nutrients or nutrient sources which all microorganisms utilize for energy. Accordingly, anaerobic bacteria are typically the organisms of choice in microbiological processes for decomposing cellulose. The use of anaerobic organisms, however, has its own set of problems.
First, many anaerobic organisms have minimal aerotolerance. That is, not only are these organisms unable to make use of atmospheric oxygen as a hydrogen acceptor, they are also highly sensitive to the presence of oxygen and may even be poisoned by it. In a large industrial operation, it is generally possible to maintain strict anaerobic conditions to protect against this lack of aerotolerance. However, maintaining these conditions may be expensive and difficult, even in these large operations. Fermentation tanks typically must be purged with inert gases and sealed off from the atmosphere. The carbon dioxide generated during fermentation typically is vented through one-way valves or the like.
Second, not all cellulose-decomposing anaerobes are capable of converting the starting material to liquid organic fuels and solvents in accordance with the "one-step" procedure described previously. Typically, the action of the organism is cellulolytic in nature, the resulting hydrolyzate being a monosaccharide such as glucose, a disaccharide such as cellobiose, or the like. A further microbiological system has to be introduced into the fermentation process in order to convert the sugars into aldehydes, ketones, alcohols, and other desired fuels and solvents.
Third, not all anaerobic bacteria have sufficient nitrogen-fixing capabilities to provide a non-distillable residue which has utility as a fertilizer.
Fourth, careful control over temperature and pH conditions may be required to insure maximum efficiency. Excessively low or high temperatures or pH's may either inactivate or kill the organisms.
Fifth, the very specificity of the microorganism or its enzymes (oftentimes an advantage in some contexts) may be a disadvantage when the nature and quality of the raw material is poorly controlled. For example, if the raw material were a mixture of agricultural wastes, waste paper, municipal sewage or garbage, or other materials equally variable in content, it is possible that not all glycosides will be broken down into simple sugars. Specificity in the ultimate products of the fermentation can also be a disadvantage if the fermentation product is essentially ethyl alcohol or a mixture containing ethyl alcohol which is easily distilled to provide the pure water-alcohol azeotrope (190 proof alcohol). This water-alcohol azeotrope is subject to heavy taxation unless it is denatured in accordance with one of the accepted denaturing formulas. Even the production and sale of denatured alcohol entails involvement in a complicated regulatory scheme which may be burdensome for the solvent or fuel manufacturer, particularly when the manufacture is being carried out on a small scale or low-volume basis.
Finally, and perhaps most important, even if the cellulose can be converted to simple liquid organic chemicals in accordance with the aforementioned "one-step" approach, the resulting products may be mixtures with little industrial utility. In the field of solvents, there is ordinarily a much greater demand for single-solvent systems which can be blended to suit the particular application. A mixture of, say, 1-butanol and acetic acid in some ratio which is arbitrarily determined by the microbiological system might be totally unsuited to most solvent applications, and separation of the alcohol from the acid might be uneconomical. In the field of fuels, the chemical identity and ratios of the components of the fuel may be less critical, provided that the overall heat of combustion is substantial, e.g. above 4 Kg-cal/g. Even in the case of relatively sensitive use of fuels (e.g. motor fuels), a mixture of various oxo- or oxy-aliphatics (including cyclo-aliphatics) can perform very adequately, provided certain volatility and antiknock requirements are met. (Ever since the internal combustion engine was invented, oxo- and oxy-aliphatics have been used successfully--the essentially pure hydrocarbon character of modern gasoline results from the ready availability of hydrocarbon fuels rather than the inability to adapt alcohols, ketones, etc. to this use.)
That is not to say that any oxo- or oxy-aliphatic mixture obtained by fermentation of cellulose can be used as a fuel. Major fermentation products may be objectionable because they are corrosive, lacking in antiknock properties, too low in volatility, unpleasant in odor, or too low in energy content, e.g. below about 4 Kg-cal/g. From the standpoint of lack of volatility, higher molecular weight aliphatic carboxylic acids, polyhydric alcohols, and mixed functional group compounds (e.g. alpha-hydroxy carboxylic acids)--all known to be products of various anaerobic fermentations--are perhaps the primary offenders. Some of these compounds are solids at room temperature. Others boil at temperatures above 200.degree. C., even though they may be liquids under normal ambient conditions. In addition, the C.sub.3 -C.sub.6 aliphatic carboxylic acids can be highly objectionable because of odor and corrosion problems.
The lower carboxylic acids, particularly formic acetic acids are objectionable for a variety of reasons. Their odors are strong, their boiling points are relatively high compared to other C.sub.1 and C.sub.2 compounds (such as the alcohols and carbonyl compounds), they are corrosive, and their energy content is well below, for example, ethyl alcohol. Yet many of the cellulolytic anaerobes produce significant amounts of carboxylic acids. Indeed, lower carboxylic acid and alpha-hydroxy carboxylic acid production are common applications of anaerobic fermentation technology.